1. Field of the Invention
The present invention relates to an antenna for seismic survey that is to a particular geometric arrangement of a plurality of vibration sensors each with appropriate sensitivity and to be used in signal correlation. The present invention antenna, for its particular geometrical arrangement, realizes a uniform spatial sampling in wavelength, thus allowing an optimal reconstruction of the waveform of the seismic vibrational signal.
In particular, the invention is directed to a specific pattern of geometrical arrangement of four vibration sensors which, used in simultaneous signal correlation between all pairs of sensors, realizes a seismic antenna achieving the highest possible efficiency in measuring the elastic and anelastic properties of the subsoil or of a construction, minimizing costs and maximizing operational capability.
2. Description of the Prior Art
The measure of the elastic parameters of the subsoil has numerous applications, including the mapping of the subsurface without drilling, the assessment of soil stability and thus the risk of failure or permanent deformation of a building structure in the long term: the latter may also be related to consolidation works of the soil beneath the existing foundation, or to the design of earthquake resistant structures as required by the soil classification schemes NEHRP (USA), Eurocode 8 (Europe) and NTC2008 (Italy), which are essentially based on the S wave velocity of a homogeneous layer equivalent to the first 30 meters of subsoil, a parameter called Vs30.
Another field of application of the measure of the elastic and anelastic parameters relates not to the soil but to engineering structures. In modern civil and mechanical engineering, the dynamic modal analysis of a structure is a fundamental tool to determine its response to transient loads. Experimentally, the dynamic modal analysis is effected through a seismic survey aimed at measuring the vibrational modes of the structure, i.e. the frequencies at which the structure mechanically resonates. This provides a measure of the elastic parameters of the structure. The anelastic parameters can be measured from the decay of the elastic ones with time after a transient.
To measure the elastic and anelastic parameters it is convenient to use indirect techniques based on measurements at the surface. Typically, indirect techniques consist of a seismic survey, either based on the analysis of the seismic vibration produced by an artificial source, or of the weak seismic noise waves which are always present everywhere due to meteoric and/or industrial activities.
More specifically, the motion sensors used by the seismic techniques are electro-mechanical vibration transducers, preferably with vibration frequency between 2 and 30 Hz, commonly known as geophones.
Conventional seismic surveys relate to the measure of the velocity of elastic P and S body waves, which allows a depth of exploration in direct relation with the ratio of the velocity of the surface layer with the layers beneath it. Since the values of this ratio are generally between 2 and 4, the depth of investigation with the traditional methods of seismic survey is limited to ½˜¼ the total length of the deployment of geophones.
Consequently, since the interest concerns at least the first 10 meters of subsoil, and more often the first 30 meters, deployments longer than at least 25, and more often longer than 100 meters, are required, with a consequently large number of geophones. The latter number is typically 16, but more often 24 (see FIG. 1) or 32, in order to overcome the difficulties of “picking” the first wave pulses. Such an experimental procedure is obviously expensive, with time-consuming setups and long operation times. In addition, it requires wide open spaces often unavailable in urban areas.
Classic seismic surveys make always use of artificial sources of waves and are based on the simple analysis of the first pulse of P and S seismic body waves, implying the use of only an infinitesimal part of the recorded signal. The situation is illustrated in FIG. 1, which shows how the classic seismic survey considers only the timing of the first pulse 1a, 1b, . . . , 1z at the various geophones to infer the alignment 2, de facto “wasting” nearly all the information contained in the recordings.
Much more effective seismic methods have been recently developed. These are based on the analysis of Rayleigh and Love elastic surface waves, which are generated by the interference of P and S waves that is induced by the combined presence of a surface and of a stratification. Such wave guide surface waves are responsible for the near surface “resonance” of the medium and constitute in practice most of a seismic recording when the sensors are placed on or near the surface.
The analysis of wave guide waves is always a non trivial operation which requires appropriate and highly complex computer codes. But is also very powerful and allows to accurately determine the profile of the P and S body wave velocities in the medium.
Such techniques of analysis for surface waves (see, for example, GB Park et al. In “Multi-channel analysis of surface waves—MASW” published in Geophysics, vol. 64, p. 800-808. 1999), use thoroughly the information contained in the seismic recordings and allow to explore the medium much more accurately and down to greater depths than the classic ones, at the same time reducing the required lengths of sensor deployments.
For surface waves is in fact valid a relationship between the wavelength and the thickness of the layer explored, so that a measure of the velocity at different frequencies allows to determine the elastic properties of the subsurface as a function of depth. In general, the components with a long wavelength L explore the medium down to greater depths, while the components with a shorter wavelength explore a shallower portion near the surface.
In “active” seismic surveys, the energizing is carried out by means of an artificial source of waves, typically a small detonation. In this way, wave fronts are generated at the surface and propagate into the subsoil.
“Passive” seismic surveys, instead, rely on the waves of seismic noise, which consists mainly of surface waves. Since noise waves have a smaller amplitude than those used in active exploration, more sensitive instruments are required.
An active—passive option exists identically also in seismic surveys aimed at the dynamic modal analysis of constructions. Namely, in the active modal analysis the structures are set into predetermined motion by hydraulic jacks, vibrators or mass impacts, while in the passive modal analysis the excitation is provided by independently existent external causes, such as seismic noise, wind, traffic, etc.
Traditionally, seismic active surveys use (see FIG. 1) in-line equally spaced deployments of geophones 3a, 3b, . . . , 3z, . . . commonly known as “arrays”, from which the wave source 4 is positioned at some distance. The wave velocity is directly measured by the time-distance curve 2 of the first pulses recorded at the various geophones. An in-line constant spaced array of geophones is thus well suited to this type of elementary analysis.
While the above-described devices fulfill their respective, particular objectives and requirements, the aforementioned patents do not describe an antenna for seismic survey with uniform spatial sampling in wavelength that allows active and/or passive seismic survey.
Therefore, a need exists for a new and improved antenna for seismic survey with uniform spatial sampling in wavelength that can be used for active and/or passive seismic survey. In this regard, the present invention substantially fulfills this need. In this respect, the antenna for seismic survey with uniform spatial sampling in wavelength according to the present invention substantially departs from the conventional concepts and designs of the prior art, and in doing so provides an apparatus primarily developed for the purpose of active and/or passive seismic survey.